Converting PostgreSQL into BigQuery including JSON Payload

Question

Can someone help me convert this query which runs on PostgreSQL to something I can run on BigQuery. I can't get my head around the JSON Payload bit

TIA

    SELECT
      (payload::json->>'duration')::int AS duration,
      a.created_at,
      a.call_sid,
      a.recording,
      a.voicemail,
      a.custom_conclusion,
      a.conclusion,
      c.first_name,
      c.last_name,
      c.phone_number 
      */
    FROM
      event as e
    LEFT JOIN
      activity as a
    ON
      a.id = e.activity_id
    LEFT JOIN
    contact AS c
    ON
      a.contact_id = c.id
    WHERE
      subscription_id = ''
      AND a.created_at > '2021-01-01 00:00:00.00000'
      AND a.created_at < '2022-01-19 23:59:59.00000'
      AND a.id IN (
      SELECT
        activity_id
      FROM
        event
      WHERE
        type = 'call_completed')
  AND e.type = 'call_completed';`